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Spooky stories: My cat is radioactive

Measuring my cat's radiation levels after radioactive iodine (I-131) treatment.

Edison being just a little radioactive

Just a little radioactive (normal level is ~0.10-0.20 uSv/h)


Several months ago, my cat, Edison, started eating and drinking a lot, yet lost a lot of weight. He was constantly anxious, very warm, and his heart was racing all of the time. We took him to the veterinarian, and after running a blood test, we found out that Edison has a benign tumor on his thyroid.

The tumor consists of the same tissue as the thyroid gland, but cannot be regulated by the body's mechanisms, releasing the thyroid hormone (T4) uncontrollably, i.e. a form of hyperthyroidism. This skyrockets the cat's metabolism, which explains all of the symptoms we witnessed. Left untreated, hyperthyroidism not only makes the cat miserable and anxious, but can lead to complications with the heart or even death. There was an observed increase in thyroid tumors in the last 20 years, but interestingly, they are mostly non-cancerous. Environmental factors, such as pollution or chemicals are a suspected cause, but there seems to be no genetic component or predisposition to it.

Although one way to address thyroid issues in cats (and people) is to administer daily medication, there is a cure for cat hyperthyroidism caused by the tumor - an injection of radioactive iodine (iodine-131 or also referred to as I-131), and the way it works is pretty neat. Active thyroid tissue absorbs/binds iodine very eagerly (also see the tangent below). Because the tumor tissue is active, but the actual thyroid is dormant (due to regulation processes), tumor absorbs most of the injected iodine, and iodine's radiation kills off the tumor tissue, leaving just a lump of dead cells. Iodine, circulated by the bloodstream, is then filtered by kidneys and expelled in urine and feces. So now you have a healthy, but radioactive, cat.

We had to limit being close to Edison for a long time for the first month, to avoid accumulating a dose - holding him was still okay, but not having him sit on the lap or next to us for longer than 10 minutes. Cat waste has to be kept and stored for 3 months until iodine decays to normal radiation levels. I purchased a geiger counter to keep track of Edison's radioactivity and to see when he reaches safe levels. Since iodine-131 emits beta and gamma particles, Edison can be "seen" through walls. Radiation falls off to background level about a meter away from the cat, although I never tested if a mass of water or anything else can block it, too.

After the treatment, Edison returned back to his chill self, gained his weight back, and even became more affectionate.

The Cat Company (HTTP-only site) - information on and treatment of cat hyperthyroidism.


Iodine-131 is produced by nuclear reactors, and could be released in case of an accident. Because it gets absorbed by the thyroid so well, it is particularly dangerous to humans, and that is why people who live around nuclear power plants get iodine supplements - to satiate the thyroid with non-radioactive iodine and prevent it from absorbing the radioactive variant. Of course, the I-131 treatment for cats is no accident and is strictly dosed.


Day	Level (uSv/h)
2	50.0
3	50.0
4	37.5
5	45.0
6	27.5
7	18.5
8	12.5
9	7.2
10	5.0
11	5.0
12	4.5
13	4.2
14	2.5
15	2.5
16	1.7
17	1.5
18	1.4
19	2.0
20	2.2
21	1.4
22	1.7
23	1.2
24	1.5
25	1.1
26	1.1
27	0.6
28	0.4
29	0.7
30	0.6
31	0.6
32	0.4
33	0.5
34	0.5
35	0.4
36	0.3
37	0.2
38	0.3
39	0.3
40	0.2
41	0.2

Graph of the data


Graph of the data with logarithmic scale



Edison being very radioactive

Very radioactive, tired Edison.

Glamour shot

Glamour shot after the defeating the tumor.